The present invention relates in general to integrated circuits. More particularly, the present invention is directed to a power management method and apparatus for changing the clock frequency in multiple, synchronous integrated circuit chips within a system.
One way of reducing energy consumption or power dissipation in an integrated circuit, such as a microprocessor, is to slow down the frequency at which circuits are switched, when performance requirements are low. There are a number of processor designs that support such frequency scaling within a single chip. Examples include the PowerPC 750FX, available from International Business Machines, Inc., and the Pentium 3, available from Intel Corp.
Several techniques are commonly used to reduce processor frequency for power management within a processor. For example, in a first technique, a processor reduces its internal clock speed independently of the rest of the system, while the bus speed is maintained at a constant rate. This is only practical if the bus frequency is a relatively low fraction of the maximum processor frequency, since there is a lower limit on the ratio of processor to bus frequency. In a second technique, a reset sequence is performed in which the processor is turned off, reconfigured along with the bus for operation at a lower frequency, and turned back on. Unfortunately, the cost of switching frequency is relatively high in this case, in terms of the latency of the transition, and so this approach does not support a fine-grained frequency adjustment scheme for managing power.
There is a need, therefore, for a method and apparatus for changing the clock frequency in multiple, synchronous integrated circuit chips within a system, while maintaining a fixed ratio of processor to bus frequency, in a seamless manner while the system continues to operate. There is also a need for a method and apparatus for changing the clock frequency in multiple, synchronous integrated circuit chips within a system, which is capable of supporting a fine-grained frequency adjustment scheme for managing power.